Adult stag beetles are extremely easy to keep as pets. While large containers are preferable, smaller ones take up less shelf space and have no negative effect on the beetles. The picture below shows how I keep some of my beetles (and a variety of other insects).

Container Any container that the beetle will fit in will probably be suitable for keeping it in, from film cases to insect cages to aquariums. The simplest method is to put the beetle in with a piece of fruit and maybe a stick. Punch holes in the lid, and you've created a home for your beetle. To give the beetle a better home though, use a slightly larger container. Line the bottom with soil or wood bits and keep the substrate moist. Larger containers will also safely hold more beetles than smaller ones, especially if there is plenty of space for them to hide in. The more resemblance of the beetles' natural habitat you make the container, the longer the beetle will probably live. Number of Beetles Per Container It is not a good idea to keep more than one stag beetle per container because they are territorial animals by nature. If two males were place together, they may fight frequently, resulting in cracked shells, broken legs and mandibles, and eventually the death of one or both beetles. Male stag beetles have the capability to inflict great damage on each other; even though this seldom happens in the wild, it often occurs under confined conditions. Placing two males together for the sake of fighting them once in a while shouldn't hurt them, but keep an eye on them to prevent them from injuring each other. Unless you plan to breed stag beetles (see Breeding and Rearing Stag Beetles),  keep the males separate from the females. Otherwise, the female may become alarmed at the presence of another beetle in her container and snip off the legs of the other beetle.

Suitable number Overcrowded

Food
Fresh fruit is very important. Beetles will not eat dried or spoiled fruit and the fruit flies attracted to the rotting fruit will be very annoying. Feed the beetles pieces of juicy fruit such as pineapple, mango, or banana, and change the fruit every couple of days to ensure its freshness. If more than one beetle are in the container, put two pieces at opposite corners of the container so the beetles won't bother each other when they feed.

Escapes
Beetles are great escape masters, so make sure the container is always tightly sealed! My earlier experiences with beetles often ended up with the beetles loose in the house. Don't underestimate their strength! A beetle can easily lift a lid that is not tightly clipped, screwed, or clamped. When you take beetles out of their container to handle, always be prepared in case it takes flight. Although stag beetles don't usually take off like that, it does happen sometimes. To land a beetle in flight, whack it with your hand and hit it to the ground. Then pick it up before it can spread its wings again. Beetles may try to fly in their containers at night, spreading their wings and flapping loudly. This is harmless, as long as there is no way they can get out. If by accident the beetle does get loose in the house, do your best to search the room the moment you discover its escape. If you can't find it, wait until dark and leave a small white lamp on in the room. Hopefully, it will be attracted to the lamp.

Death
Most adult stag beetles don't live through the winter, so they will probably die at the end of summer or autumn. This is when you can make them into specimens (see Making of Beetle Specimens). Beetles sometimes feign death as a means of escape, so be sure that they are really dead before you pin them! Beetles may also be kept fresh in the refrigerator for months if you don't have time to make them into specimens the moment they die.

Fighting Beetles
Fighting beetles is fine, but don't be cruel to them. They are living creatures and are able to suffer and feel pain. See also Beetle Fight!